This invention relates to a method for preparing alkylhalosilanes. More specifically, this invention relates to the use of arsenic compounds and arsenic alloys as a replacement for tin as a catalyst in the direct reaction of silicon and an alkyl halide to produce alkylhalosilanes. The benefits to be derived from using the instant invention are increased alkylhalosilane yields, selectivity of certain alkylhalosilanes over other, less preferred alkylhalosilanes, and overall higher utilization of raw materials in the reaction
Halosilanes and organohalosilanes are well known reactive chemical intermediates used extensively in the semiconductor and silicones industries, respectively. Halosilanes and organohalosilanes are produced primarily by the direct reaction of silicon with the corresponding hydrogen halide or organic halide. Hereafter in this application the term "the direct reaction" will refer either to the reaction of silicon with a hydrogen halide or the reaction of silicon with an organic halide. The direct reaction to form the halosilanes has been known since the work of Buff and Wohler in 1857 and Combes in 1896. The direct reaction to form organohalosilanes was first disclosed by Rochow and his co-workers, beginning in the mid-1940'". The direct reaction for producing alkylhalosilanes is well-known and has been refined and modified in many ways since the early work of Rochow.
It should be noted that the direct reaction for producing alkylhalosilanes produces a distribution of silanes containing all combinations of alkyl, halogen, hydrogen, and other organic groups. As an example, the direct reaction of methyl chloride with silicon can produce a wide range of silane materials, for example, silane, trichlorosilane, tetrachlorosilane, tetramethylsilane, methytrichlorosilane, dimethyldichlorosilane, trimethylchlorosilane, methyldichlorosilane, and dimethylchlorosilane.
In the modern manufacture of silicones, diorganodihalosilanes, dimethyldichlorosilane in particular, constitute the bulk of the silane intermediates that are processed to produce the siloxane intermediates that are utilized in most silicone products. When one considers that hundreds of millions of pounds of silanes are produced annually worldwide to support the commercial manufacturing of silicones, it can be appreciated that even small increments of improvement in selectivity in crude product distribution and of improvement in raw material efficiency have a significant impact upon these manufacturers. As an example, assuming a manufacturer produces ten million pounds of dimethyldichlorosilane annually, a two to four weight percent reduction in the silicon raw material requirement would have a significant economic impact and would be quite attractive to the manufacturer.
For the purposes of the instant invention, the efficiency of utilizing raw materials is tracked by the amount of silicon charged that is converted to silanes. This conversion of silicon to silanes will be hereafter denoted as "% Si conversion". Those skilled in the art are interested in the selectivity of the direct reaction, particularly the formation of diorganodihalosilanes compared to less preferred organohalosilanes and halosilanes. In the preparation of methylchlorosilanes, a measure of selectivity of particular intereat is the ratio of methyltrichlorosilane (Me) to dimethyldichlorosilane (Me.sub.2). For the purposes of the instant invention, this measure of selectivity will be denoted by the ratio "Me/Me.sub.2 ". In the literature, this ratio is often referred to as "M/M.sub.2 " or "T/D". Therefore, an increase in the Me/Me.sub.2 ratio indicates that there is an decrease in the output of the more preferred dimethyldichlorosilane, Conversely, a decrease in the ratio indicates that there is an increase in the output of the more preferred dimethyldichlorosilane.
Rochow, U.S. Pat. No. 2,380,995, issued Aug. 7, 1945, showed contacting methyl chloride vapor with silicon at about 300.degree. C. yielded a silane mixture that was predominantly methyltrichlorosilane and dimethyldichlorosilane at 52 and 14,5 weight percent, respectively. This results in a Me/Me.sub.2 ratio of 3.6. Rochow also discloses the use of a 50/50 weight silicon-copper alloy and the use of metallic catalysts other than copper, such as nickel, tin, antimony, manganese, silver, and titanium. The physical forms and the amounts of these catalysts are not disclosed by Rochow.
Rochow and Patnode, U.S. Pat. No. 2,380,996, issued Aug, 7, 1945, and Patnode, U.S. Pat. No. 2,380,997, issued Aug, 7, 1945, disclose the preparation of a contact mass for the direct reaction, the method comprising the subjecting of a mixture of silicon, copper, or other metallic catalysts to a reducing atmosphere during firing. Rochow and Patnode and Patnode also disclose the use of nickel, tin, antimony, manganese, silver, and titanium.
Rochow and Gilliam, U.S. Pat. No. 2,383,818, issued Aug, 28, 1945, discloses the use of contact masses comprising silicon and an oxide of copper. Also, included are copper compounds which are readily converted to the oxides, such as copper nitrate. Overall yield and the Me/Me.sub.2 ratio are not disclosed.
Ferguson and Sellers, U.S. Pat. No. 2,443,902, issued June 22, 1948, disclose an attempt to increase the yield of dialkyldihalosilanes from the direct reaction. Ferguson and Sellers discloses the reaction between an alkyl halide and silicon in the presence of a finely divided cupreous catalyst, having as a principal constituent, friable metallic copper core surrounded by protective surface films of cuprous oxide, inhibiting oxidation in air, said films being relatively thin compared with the size of the enclosed copper cores. Two examples showed that Me/Me.sub.2 ratios of 0.18 and 0.77, respectively were obtained. For these two examples, the percent of silicon converted that was converted to dimethyldichlorosilane was 82.4 and 65.6, respectively.
Gilliam, U.S. Pat. No. 2,464,033, issued March 8, 1949 discloses the use of copper halides, in addition to copper metal and copper oxides, as catalysts in the direct process. Further, Gilliam discloses the use of "promoters" such as zinc, or zinc halides, or their mixtures. Favorable Me/Me.sub.2 ratios in a range from about 0.20 to 0.40 were demonstrated. Corresponding figures on the percent of silicon converted that was converted to dimethyldichlorosilane were in a range from about 45 to 65.
Nitzsche, U.S. Pat. No. 2,666,775, issued Jan, 19, 1954, discloses the use of alloys of silicon with either copper or iron or both which were activated with chlorides. The results were a Me/Me.sub.2 ratio of 0.90.
From an abstract published in Chemical Abstracts, 58 (1963), 13995, a method utilized by Golubstov et al., in a U.S.S.R. Patent publication, published Oct, 31, 1962, discloses the use of arsenic as an accelerator to increase the output of dimethylchlorosilane, (CH.sub.3).sub.2 HSiCl, from the reaction of a silicon-copper alloy with methyl chloride. No mention is made of the output of dimethyldichlorosilane. Further, Golubstov et al., does not disclose the combination of arsenic with copper or copper compounds, zinc or zinc compounds, and phosphorous or phosphorous compounds, as does the instant invention.
Rossmy, U.S. Pat. No. 3,069,452, issued Dec, 18, 1962, discloses the use of a brittle, grindable silicon-copper alloy as a catalyst for the direct reaction of silicon and an alkyl halide. The use of this new copper catalyst results in Me/Me.sub.2 ratios as low as 0.13.
Lobusevich et al., Zhurnal Obshchei Khimii, (August, 1964), 34:8, pp. 2727-2729, discloses that arsenic at a concentration of 0.05 to 1.0 weight percent is a promoter in raising the total and selective activities of silicon-copper alloys in the direct reaction with methyl chloride to produce methylchlorosilanes, Lobusevich et al., in FIG, 4 summarizes the impact of arsenic when the direct reaction is run at a temperature of 320.degree. C. Over the range of arsenic content studied, the content of dimethyldichlorosilane in the reaction product is increased from approximately 50 weight percent to approximately 65 weight percent. No mention is made of the Me/Me.sub.2 ratio. Further, no mention is made of the percentage of silicon converted to dimethyldichlorosilane.
Rossmy, German Pat. No, 1,165,026, published March 12, 1964, discloses the use of silicon-copper alloys for the preparation of methylchlorosilanes. The silicon-copper alloys were prepared by sintering a finely ground silicon-copper with an additive such as phosphorous, arsenic, antimony, indium, thallium, and gallium in a stream of hydrogen at temperatures of about 1000.degree. C. Example 3 of Rossmy outlines the use of arsenic as the additive to the silicon-copper alloy. More specifically, ferrosilicon containing 95.5 percent silicon was was sintered in a stream of hydrogen at 1030.degree. C. with Cu.sub.3 Si and arsenic, wherein the arsenic content was 0.09 weight percent or 9OO parts per million (ppm). The resulting alloy was reacted with methyl chloride at 290.degree. C. to yield a reaction product mixture containing 77.5 percent dimethyldichlorosilane and having a Me/Me.sub.2 ratio of approximately 0.17. No mention is made of the percentage of silicon converted to methylchlorosilanes. Special note should be taken of the manner in which the arsenic was introduced into the reaction of the methyl chloride and the silicon-copper mass.
Zock, U.S. Pat. No. 3,446,829, issued May 27, 1969, discloses the use of a cadmium promoter with a copper or silver catalyst with silicon in the direct reaction. The advantages forwarded by Zock are increased rate of reaction; increased selectivity for the formation of dimethyldichlorosilane; and high silicon conversion rate.
U.S. Pat. No. 4,218,387, issued Aug. 19, 1980. discloses the preparation of catalytic copper in terms of its particle size and copper (I) oxide content to give higher yields and greater selectivity. Maas et al., shows an example of a Me/Me.sub.2 ratio as low as 0.90. No mention is made of the percentage of silicon converted to methylchlorosilanes.
Downing et al., European Patent Application No, 0 028 009 A2, published, May 6, 1981, discloses a copper-catalyzed silicon reaction mass for the production of methylchlorosilanes which comprises free-flowing powders or particles of silicon metal having spots of copper-silicon alloy substantially uniformly distributed on the surface of the silicon particles. Zinc powder is optionally used in the reaction mass. Downing et al., provide only one example of the use of this reaction mass to produce methylchlorosilanes. A Me/Me.sub.2 ratio of approximately 0.07 is reported by Downing et al. No reference is made to the percentage of silicon converted to dimethyldichlorosilane.
Ward et al., U.S. Pat. No. 4,487,950, issued Dec, 11, 1984, discloses the preparation of methylchlorosilanes from the direct reaction of methyl chloride and particulated silicon which has been contacted with a mixture of partially oxidized copper catalyst and copper formate. Ward et al., discloses a Me/Me.sub.2 ratio as low as 0.09.
Ward et al., U.S. Pat. No. 4,500,724, issued Feb, 19,1985, discloses the use of tin as a co-catalyst with copper and zinc, especially when copper is in the form of copper chloride. Ward et al., claim that the reaction rate and the selectivity of the direct reaction of silicon with an alkyl halide are achieved when attention is paid to the critical weight percent of copper relative to silicon and the critical weight ratios of tin and zinc are employed relative to copper. Ward et al., in Table 1 of Example 1 summarizes the impact of tin upon the Me/Me.sub.2 ratio. Ward et al., demonstrate that tin is needed, in combination with copper and zinc to minimize the Me/Me.sub.2 ratio.
Hashiguchi, U.S. Pat. No. 4,503,165, issued March 5, 1985, discloses the use of hydroxides of Period IV metals, having atomic numbers between 24 and 30, as catalysts in conjunction with ground cupreous particulates in the preparation of alkyl and arylhalosilanes. Shoepe and Hashiguchi, U.S. Pat. No. 4,504,596, issued March 12, 1985, discloses the use of hydrated refractory oxides, such as hydrated alumina, with a major portion of cuprous and cupric oxides and a minor portion of copper in the preparation of alkyl and arylhalosilanes.
Prud'Homme European Patent Publication No, 0,138,678 A2, published Apr, 24, 1985, discloses a process for improving dimethylchlorosilane selectivity and maximum silicon conversion, the process comprising the reacting of a solid contact mass of silicon and a catalyst including copper or a copper compound with the catalyst additionally including tin or antimony or compounds of tin or antimony and cesium or a cesium compound. Prud'Homme also discloses that zinc or zinc compounds may also be included in the catalyst, and further that zinc may be partially replaced by another metal such as aluminum, cadmium, manganese, nickel, or silver. Prud'Homme discloses that it is necessary for either tin or antimony to be present in the catalyst for the beneficial effects of the invention to be obtained. Prud'Homme further discloses that high selectivity can be obtained whenever the catalyst is used at a temperature of 330-350.degree. C. Prud'Homme presents 9 examples applying the catalyst of the invention in which Me/Me.sub.2 ratios ranging from 0.04 to 0.08 and silicon conversions ranging from 74 to 89 percent ar reported. In a comparative example in which tin or antimony is not present the Me/Me.sub.2 ratio is 0.08 and a silicon conversion of 4 percent. Thus tin or antimony must be present to have both high selectivity and high silicon conversion
Prud'homme, European Patent Publication No, 0 138 679, published Apr, 24, 1985, makes a similar disclosure in which lithium, sodium, potassium, and rubidium metal and compounds of these metals are utilized with tin or antimony. Me/Me.sub.2 ratios ranging from approximately 0.09 to 0.13 are reported
Prud'homme, European Patent Publication No, 0 194 214, published Sept, 10, 1986, makes a similar disclosure in which beryllium, magnesium, and calcium metal and compounds of these metals are utilized with tin or antimony. A Me/Me.sub.2 ratio of 0.09 is reported.
Prud'Homme, U.S. Pat. No. 4,645,851, issued Feb, 24, 1987, discloses as similar invention in which barium or strontium are added as metals or compounds thereof, in place of cesium or cesium compounds, When the catalyst of the invention was applied, results reported showed Me/Me.sub.2 ratios ranging from 0.05 to 0.10 and silicon conversions ranging from 52 to 70 percent. When the catalyst was applied without tin the Me/Me.sub.2 ratio was 0.19 and the silicon conversion was not reported.
Ritzer et al., United Kingdom Patent Application GB No, 2,153,697, published Aug, 29, 1985, discloses catalysts for the production of organohalosilanes comprising copper and copper oxides, tin or tin-containing compounds, and aluminum or aluminum-containing compounds. Me/Me.sub.2 ratios ranging from approximately 0.09 to 0.10 and silicon conversion ranging from about 28 to 45 percent were reported.
Halm et al., U.S. Pat. No. 4,602,101, issued July 22, 1986, discloses a method for controlling a process for the manufacture of alkylhalosilanes, said process comprising contacting an alkyl halide with metallurgical grade silicon, at a temperature of 250-350.degree. C., in the presence of tin or tin compounds, and copper or copper compounds, wherein there is present as a promoter phosphorous or phosphorous-containing compounds. The benefits of the invention of Halm et al., are increased halosilane yields, increased selectivity of dialkyldihalosilanes over other, less preferred alkylhalosilanes, and overall higher utilization of raw materials. Me/Me.sub.2 ratios as low as 0.04 and silicon conversions as high as 87 percent were reported. As in most of the above references, tin is a necessary component of the catalyst to realize the benefits of the invention of Halm et al.
Lewis and Childress, European Patent Publication No, 0 191 502 A2, published Aug, 20, 1986, discloses an improved direct process wherein the tin and zinc promoter content in the catalyst are controlled at a promoter to tin ratio of 10 to 250, by weight, Me/Me.sub.2 ratios down to as low as 0.035 were reported. However silicon conversion was very low, normally less than 20 percent.
None of the above references show that a low Me/Me.sub.2 ratio can be attained with high conversion of silicon to methylchlorosilanes without the use of tin or antimony in the contact mass for the direct reaction. Further, none of the above references have shown that arsenic or compounds of arsenic when added to the direct reaction of silicon and methyl chloride will allow the minimum Me/Me.sub.2 ratio and maximum silicon conversion to methylchlorosilanes that is obtained when tin or tin compounds are used in the direct reaction.